Computed NMR shielding increments over unsaturated five-membered ring heterocyclic compounds as a measure of aromaticity

The GIAO-HF method within Gaussian 03 was used to calculate the isotropic shielding value of the proximal hydrogen of a diatomic hydrogen probe at various distances (2.5 Å, 3.0 Å and 4.0 Å) above the plane of 15 conjugated five-membered ring heterocyclic compounds: pyrrole, furan, thiophene, and phosphole and their 2- and 3-nitrogen analogs. Subtraction of the isotropic shielding value of diatomic hydrogen by itself from each of these isotropic shielding values gave the shielding increment (Δσ) for each probe position. Plotting Δσ against Cartesian coordinates of the probe position allowed determination of the computed through-space shielding increment surfaces for these compounds. Substantial shielding was observed above the center of each ring, as expected for aromatic compounds. The magnitude of the shielding increment at 2.5 Å above the ring center (Δσ2.5) correlated reasonably well with other established methods of assessing aromaticity, including geometric (HOMA, harmonic oscillator model of aromaticity), energetic (ASE, aromatic stabilization energy), and magnetic (NICS, nucleus-independent chemical shift) criteria.